Steel sheet, plated steel sheet, method of production of hot-rolled steel sheet, method of production of cold-rolled full hard steel sheet, method of production of steel sheet, and method of production of plated steel sheet

ABSTRACT

A steel sheet, a plated steel sheet, and methods for producing a hot-rolled steel sheet, a cold-rolled full hard steel sheet, and a steel sheet. The steel sheet has a specified composition and a microstructure including 0 to 80% of polygonal ferrite and 20 to 100% of a total of martensite, bainite, and residual austenite in terms of an area ratio within 20 μm of the steel sheet surface. The content of Mn in martensite present within 20 μm of the steel sheet surface ([Mn] SM ) and the content of Mn in a bulk ([Mn] B ) satisfy [Mn] SM /[Mn] B ≤1.5. At a location 300 μm from the steel sheet surface, an area ratio of the martensite is in a range of 20 to 50%.

TECHNICAL FIELD

This application relates to steel sheets, plated steel sheets, a methodof production of hot-rolled steel sheets, a method of production ofcold-rolled full hard steel sheets, a method of production of steelsheets, and a method of production of plated steel sheets.

BACKGROUND

High strength of steel sheet for use in automobile components isdemanded from the viewpoint of improving collision safety and fuelenergy of automobiles. However, strengthening of materials generallyleads to deterioration in workability. For this reason, development ofsteel sheet excellent in both strength and workability is required. Inparticular, high-strength steel sheets having tensile strength (whichmay be hereinafter referred to as TS) of over 930 MPa, are oftensubjected to bending-based working in a straight shape like members androcker components because of difficulty in forming, so there is a needfor steel sheets excellent in bendability. Rust prevention is requiredfor these components under corrosive conditions.

A hard microstructure in the vicinity of the surface layer promotesoccurrence of voids at the time of bending and becomes a starting pointof cracking. Therefore, it is extremely important, from the viewpoint ofimprovement of bendability, to suppress occurrence of voids related tothe hard microstructure in high-strength steel sheets having TS of over980 MPa.

In this regard, PTL 1 discloses a technique related to a hot-dipgalvanized steel sheet excellent in bendability by controlling hardnessof tempered martensite and ferrite.

PTL 2 discloses a technique related to a hot-rolled steel sheetexcellent in bendability by reducing martensite while hardening ferrite.

PTL 3 discloses a technique related to a hot-dip galvanized steel sheetexcellent in bendability by lowering the strength in the vicinity of thesurface layer.

CITATION LIST Patent Literature

PTL 1: JP-A-2010-275627

PTL 2: JP-A-2006-161111

PTL 3: JP-A-2015-34334

SUMMARY Technical Problem

However, PTL 1 does not consider micro cracks occurring on the steelsheet surface, and needs further improvement. Despite the fact that alarge amount of Si and Mn, which inhibit plating capabilities, iscontained, countermeasures are not disclosed in a manufacturingtechnology, so that it is conceivable that rust prevention becomesinsufficient due to occurrence of non-plating.

PTL 2 related to the hot-rolled steel sheet does not consider bendingworkability in a state where plating is applied and micro cracks on thebent surface in the state, and needs further improvement.

PTL 3 does not consider micro cracks occurring on the steel sheetsurface, and needs further improvement.

The disclosed embodiments have been made under these circumstances, andit is an object of the disclosed embodiments to provide a plated steelsheet excellent in bendability and plating capabilities with tensilestrength of 980 MPa or more, and a method of production of such platedsteel sheet. The disclosed embodiments are also intended to provide asteel sheet needed to obtain the plated steel sheet, methods ofproduction of a hot-rolled steel sheet and a cold-rolled full hard steelsheet needed to obtain the plated steel sheet, and a method ofproduction of the steel sheet.

Solution to Problem

The present inventors conducted intensive studies to find a solution tothe foregoing problems. The studies revealed the following findings.

It was found that voids related to martensite in the vicinity of thesurface layer of the steel sheet are strongly influenced by hardnessdifference between martensite and other phases, the fraction ofmartensite, and the Mn content in martensite. When Mn-enriched parts areformed on the steel sheet surface, tiny non-plating area generates fromthe part.

It was also found that non-plating was not generated and excellentbendability was exhibited in the steel sheet having TS of over 980 MPaunder a condition in which the content of Mn in martensite present in arange of 20 μm in sheet thickness direction from the steel sheet surface([Mn]_(SM)) and the content of Mn in bulk ([Mn]_(B)) are set to satisfy[Mn]_(SM)/[Mn]_(B)≤1.5 after performing optimizations of the compositionand microstructure of steel.

The disclosed embodiments are based on these findings, and theconfiguration is as follows.

[1] A steel sheet of a composition including, in mass %, C: 0.05 to0.25%, Si: 1.0% or less, Mn: 1.5 to 4.0%, P: 0.100% or less, S: 0.02% orless, Al: 1.0% or less, N: 0.001 to 0.015%, one or more selected fromthe group consisting of Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%, Mo:0.005 to 0.500%, and the balance being Fe and unavoidable impurities,

wherein polygonal ferrite is 0 to 80%, and a total of martensite,bainite, and residual austenite is 20 to 100% in terms of an area ratioin a range of 20 m in a sheet thickness direction from a steel sheetsurface,

wherein the content of Mn in martensite present in the range of 20 μm inthe sheet thickness direction from the steel sheet surface ([Mn]_(SM))and the content of Mn at a position (in a bulk) of ¼-thickness of thesteel sheet toward a center in the sheet thickness direction from thesteel sheet surface ([Mn]_(B)) satisfies [Mn]_(SM)/[Mn]_(B)≤1.5, andwherein the martensite has an area ratio of 20 to 50% at a position 300μm in the sheet thickness direction from the steel sheet surface.

[2] The steel sheet according to item [1], wherein the compositionfurther includes at least one selected from the group consisting of, inmass %, Cr: 0.005 to 2.000%, V: 0.005 to 2.000%, Cu: 0.005 to 2.000%,Ni: 0.005 to 2.000%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, REM:0.0001 to 0.0050%, Sb: 0.0010 to 0.1000%, and Sn: 0.0010 to 0.5000%.

[3] A plated steel sheet comprising a plating layer on a surface of thesteel sheet of item [1] or [2].

[4] The plated steel sheet according to item [3], wherein the platinglayer is a hot-dip galvanized layer or a hot-dip galvannealed layer.

[5] A method for producing a hot-rolled steel sheet, including:

hot rolling the slab of the composition of item [1] or [2], hot-rollingincluding finish-rolling under conditions that a temperature from asecond-last pass to the last pass is 800 to 950° C., a cumulativerolling reduction from the second-last pass to the last pass is 10 to40%, and a rolling reduction of the last pass is 8 to 25%;

cooling with start in a range of 0.5 to 3.0 sec after an end offinish-rolling cooling at an average cooling rate of 30° C./s or more ina temperature range of 600 to 720° C.; and

coiling at a temperature of 590° C. or lower.

[6] A method for producing a cold-rolled full hard steel sheet,including: cold rolling the hot-rolled steel sheet obtained by themethod of item [5] at a cold-rolling ratio of 20% or more.

[7] A method for producing a steel sheet, comprising annealing thehot-rolled steel sheet obtained by the method of item [5] or thecold-rolled full hard steel sheet obtained by the method of item [6],

wherein the hot-rolled steel sheet or the cold-rolled full hard steelsheet is heated to a temperature of 730 to 900° C. and is then cooled toa cooling stop temperature of 400 to 590° C. at an average cooling rateof 5 to 50° C./s, and

wherein the hot-rolled steel sheet or the cold-rolled full hard steelsheet is retained for 10 to 1,000 sec in a temperature range of 730 to900° C. at the time of the heating and the cooling, and is retained for1,000 sec or less in a temperature range of 400 to 590° C.

[8] The method according to item [7], wherein a dew point in atemperature range of 730 to 900° C. is −40° C. or lower.

[9] A method for producing a plated steel sheet, including:

plating the steel sheet obtained by the method of item [7] or [8].

As used herein, “high-strength” means a TS of 980 MPa or more,“excellent bendability” means that an R/t (to be described below) is 2.5or less, and “excellent plating capabilities” means that non-platinghaving a diameter of 0.5 mm or more is not recognized when the surfaceof the galvanized steel sheet surface is observed with a magnifier at amagnification of 10 times. From the viewpoint of obtaining excellentbendability intended in the disclosed embodiments, the TS is preferablyless than 1,180 MPa, more preferably 1, 175 MPa or less.

Advantageous Effects

The disclosed embodiments enable providing a plated steel sheet havingexcellent bendability and plating capabilities with tensile strength of980 MPa or more. Because of these properties, the plated steel sheet ofthe disclosed embodiments is preferred as material of automobilecomponents.

The steel sheet, and the methods of production of hot-rolled steelsheets, cold-rolled full hard steel sheets, and steel sheets of thedisclosed embodiments contribute to improving the collision safety andthe fuel consumption of automobiles as an intermediate product forobtaining the plated steel sheet having desirable properties as abovementioned, or as methods for producing such intermediate products.

DETAILED DESCRIPTION

An embodiment of the disclosed embodiments is described below.

The disclosed embodiments include a steel sheet, a plated steel sheet, amethod of production of hot-rolled steel sheets, a method of productionof cold-rolled full hard steel sheets, a method of production of steelsheets, and a method of production of plated steel sheets. The followingfirstly describes how these are related to one another.

The steel sheet of the disclosed embodiments is also an intermediateproduct for obtaining the plated steel sheet of the disclosedembodiments. The steel sheet of the disclosed embodiments is producedfrom a starting steel material such as a slab through producingprocesses that produce a hot-rolled steel sheet, a cold-rolled full hardsteel sheet. Further, the plated steel sheet of the disclosedembodiments is obtained from plating of the steel sheet.

The method for producing a hot-rolled steel sheet of the disclosedembodiments is a part of the foregoing processes that produces ahot-rolled steel sheet.

The method for producing a cold-rolled full hard steel sheet of thedisclosed embodiments is a part of the foregoing processes that producesa cold-rolled full hard steel sheet from the hot-rolled steel sheet.

The method for producing a steel sheet of the disclosed embodiments is apart of the foregoing processes that produces a steel sheet from thecold-rolled full hard steel sheet.

The method for producing a plated steel sheet of the disclosedembodiments is a part of the foregoing processes that produces a platedsteel sheet from the steel sheet.

Because of these relationships, the hot-rolled steel sheet, thecold-rolled full hard steel sheet, and the steel sheet, plated steelsheet has a common composition. Likewise, the steel sheet and the platedsteel sheet share the same steel microstructure. The following describessuch common features first, followed by the steel sheet, the platedsteel sheet, and the methods of production of these members, in thisorder.

<Composition of Hot-Rolled Steel Sheet, Cold-Rolled Full Hard SteelSheet, Steel Sheet, and Plated Steel Sheet>

The hot-rolled steel sheet, the cold-rolled full hard steel sheet, thesteel sheet, and the plated steel sheet have a composition, in mass %,C: 0.05 to 0.25%, Si: 1.0% or less, Mn: 1.5 to 4.0%, P: 0.100% or less,S: 0.02% or less, Al: 1.0% or less, N: 0.001 to 0.015%, at least oneselected from Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%, and Mo: 0.005 to0.500%, and the balance being Fe and unavoidable impurities.

The composition may further contain, in mass %, at least one selectedfrom Cr: 0.005 to 2.000%, V: 0.005 to 2.000%, Cu: 0.005 to 2.000%, Ni:0.005 to 2.000%, B: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, REM:0.0001 to 0.0050%, Sb: 0.0010 to 0.1000%, and Sn: 0.0010 to 0.5000%.

The following describes each composition. In the following description,“%” representing the content of each composition means “mass %”. Inaddition, “amount” means “content amount”.

C: 0.05 to 0.25%

Carbon (C) is an element that is effective for increasing TS by formingmartensite or bainite. When the C content is less than 0.05%, such aneffect cannot be sufficiently obtained, and TS of 980 MPa or more cannotbe obtained. On the other hand, when C content exceeds 0.25%, martensiteis hardened and bendability significantly deteriorates. For this reason,the C content is 0.05 to 0.25%. The C content is preferably more than0.06%, more preferably 0.07% or more. The C content is preferably 0.22%or less, more preferably 0.20% or less.

Si: 1.0% or Less

Silicon (Si) is an element that is effective for increasing TS throughsolid-solution strengthening of steel, but is also an element thatsignificantly inhibits plating capabilities and leads to non-plating.The upper limit of the Si content is 1.0% in the disclosed embodiments.For this reason, the Si content is 1.0% or less, preferably 0.8% orless, more preferably 0.6% or less. The lower limit is not particularlyspecified, but is preferably 0.005% or more from the viewpoint ofworkability.

Mn: 1.5 to 4.0%

Manganese (Mn) is an element that is effective for increasing TS throughformation of martensite and bainite. When the Mn content is less than1.5%, such an effect cannot be sufficiently obtained, and polygonalferrite is excessively formed to cause a decrease of Tensile Strength(TS) and the deterioration of bendability. On the other hand, when theMn content exceeds 4.0%, the steel becomes brittle and the bendabilityof the disclosed embodiments cannot be obtained. For this reason, the Mncontent is 1.5 to 4.0%. Preferably, the Mn content is 2.0% or more. TheMn content is preferably 3.5% or less.

P: 0.100% or Less

Phosphorus (P) makes grain boundaries brittle and deterioratesbendability. It is accordingly preferable to contain phosphorus in aslow as possible. In the disclosed embodiments, phosphorus may becontained in an amount of at most 0.100%. For this reason, the P contentis 0.100% or less. The lower limit is not particularly specified, but Pof about 0.001% may be inevitably mixed in the steel. For this reason,the P content is preferably 0.001% or more because the productionefficiency is lowered in the case of being less than 0.001%.

S: 0.02% or Less

Sulfur (S) increases inclusions and deteriorates bendability. It isaccordingly preferable to contain sulfur in as low as possible. In thedisclosed embodiments, however, sulfur may be contained in an amount(“amount” means “content amount”) of at most 0.02%. For this reason, theS content is 0.02% or less. The lower limit is not particularlyspecified, but is preferably 0.0005% because the production efficiencyis lowered in the case of being less than 0.0005%.

Al: 1.0% or Less

Aluminum (Al) acts as effective as a deoxidizing agent, and ispreferably contained at the time of deoxidation. When a large amount ofAl is contained, a large amount of polygonal ferrite is formed to causethe deterioration of TS and bendability. In the disclosed embodiments,however, the upper limit of the Al content is 1.0%. For the reason, theAl content is 1.0% or less. Preferably, the Al content is 0.010% ormore. Al content is preferably 0.50% or less.

The total content of Si and Al is preferably less than 0.8% in terms ofplating capabilities. Even in the case of being less than 0.8%, theeffect of the disclosed embodiments can be sufficiently obtained.

N: 0.001 to 0.015%

Nitrogen (N) is an element that forms nitrides such as AlN, and iseffective at refining the grain size. Nitrogen needs to be contained inan amount of 0.001% or more to obtain this effect. On the other hand,when the N content is more than 0.015%, coarse nitrides occur, theeffect of making fine grains becomes weaker, and bendabilitydeteriorates due to coarse nitrides. For this reason, the N content is0.001 to 0.015%, preferably 0.002% or more, more preferably 0.003% ormore. Preferably, the N content is 0.012% or less, more preferably0.010% or less.

At least One selected from Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%, andMo: 0.005 to 0.500%

Titanium (Ti), niobium (Nb), and molybdenum (Mo) are effective elementsfor forming carbides at the time of annealing to refine themicrostructure and suppress cracks during bending due to precipitationhardening and to improve bendability. In order to obtain such an effect,at least one selected from Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%, andMo: 0.005 to 0.500% should be contained. On the other hand, whencontained in amounts above the above-described specified limits, theseelements may cause coarsening of carbides and an adverse effect such asdeterioration in bendability. For this reason, the contents of Ti, Nb,and Mo, when contained, are Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%,and Mo: 0.005 to 0.500%. Preferably, the Ti content is 0.010% or more,the Nb content is 0.010% or more, and the Mo content is 0.010% or more.Preferably, the Ti content is 0.060% or less, the Nb content is 0.080%or less, and the Mo content is 0.300% or less.

The balance is Fe and unavoidable impurities.

The steel sheet of the disclosed embodiments has the basic compositiondescribed above. The composition may also appropriately contain one ormore of the following optional elements, as required.

At least One Selected from Cr: 0.005 to 2.000%, V: 0.005 to 2.000%, Cu:0.005 to 2.000%, Ni: 0.005 to 2.000%, B: 0.0001 to 0.0050%, Ca: 0.0001to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.1000%, and Sn:0.0010 to 0.5000%

Cr, V, and Cu are elements that are effective for forming martensite andbainite to increase the strength of steel. In order to obtain such aneffect, the contents of these elements are preferably Cr: 0.005% ormore, V: 0.005% or more, and Cu: 0.005% or more. When the contents ofCr, V, and Cu are more than 2.000%, 2.000%, and 2.000%, respectively,deterioration of bendability and non-plating due to inhibition ofplating capabilities are caused.

The contents of these elements, when contained, are Cr: 0.005 to 2.000%,V: 0.005 to 2.000%, and Cu: 0.005 to 2.000%.

Nickel (Ni) is an element that is effective for forming martensite andbainite and increasing the strength of steel. In order to obtain thiseffect, the Ni content is preferably 0.005% or more. When the Ni contentis more than 2.000%, the properties of martensite changes, resulting indeterioration of bendability. The content of Ni, when contained, is0.005 to 2.000%.

Boron (B) is an element that is effective for improving hardenability ofthe steel sheet, forming martensite and bainite, and increasing thestrength of steel. In order to obtain such an effect, the B content ispreferably 0.0001% or more. On the other hand, when the B content ismore than 0.0050%, inclusions increase and bendability deteriorates. Forthis reason, the content of B, when contained, is 0.0001 to 0.0050%.

Ca and REM are elements that are effective for improving bendability bycontrolling the form of inclusions. In order to obtain such an effect,the content of these elements are preferably Ca: 0.0001% or more andREM: 0.0001% or more. On the other hand, when the contents of Ca and REMare more than 0.0050%, respectively, the amount of inclusions increasesand bendability deteriorates. For this reason, the contents of theseelements, when contained, are Ca: 0.0001 to 0.0050% and REM: 0.0001 to0.0050%.

Sb and Sn are elements that are effective for inhibiting reactions suchas denitrogenation and deboronation and inhibiting strength reduction ofthe steel. In order to obtain such an effect, the contents of theseelements are preferably Sb: 0.0010% or more and Sn: 0.0010 or more. Onthe other hand, when the Sb content is more than 0.1000% and the Sncontent is more than 0.5000%, bendability deteriorates due toembrittlement of grain boundaries. For this reason, the contents ofthese elements, when contained, are Sb: 0.0010 to 0.1000% and Sn: 0.0010to 0.5000%.

In the steel sheet of the disclosed embodiments, additional elements Zr,Mg, La, and Ce may be contained in a total amount of at most 0.002%.

[Mn]_(SM)/[Mn]_(B) being ratio of content of Mn in martensite present inrange up to 20 m in sheet thickness direction from steel sheet surface([Mn]_(SM)) to content of Mn in bulk ([Mn]_(B)): 1.5 or Less

When the [Mn]_(SM)/[Mn]_(B) is more than 1.5, bendability deterioratesand plating capabilities also deteriorates. Although the mechanism ofthe deterioration in bendability is not clear, it is assumed that voidformation is promoted due to a sharp Mn concentration gradient at theinterface during deformation and cracks easily occurs when the contentof Mn in hard martensite increases and the difference in Mn content fromother microstructures becomes large. For this reason, the[Mn]_(SM)/[Mn]_(B) is 1.5 or less, preferably 1.3 or less.

The amount of Mn in the bulk means the Mn content at a location of ¼toward the center in the sheet thickness direction from the steel sheetsurface.

The [Mn]_(SM) and the [Mn]_(B) were measured by the following method. Asample was cut from the annealed steel sheet, a cross section was takenthrough the thickness parallel to the rolling direction, and amicrostructure of the cross section was observed. In the range to thelocation of 20 μm in the sheet thickness direction from the steel sheetsurface, EDX analysis was performed on the central part of themicrostructure corresponding to white and light gray parts excludingcarbides in each of ten fields of view, and the average Mn content (Mncontent in martensite) was calculated, which was defined as [Mn]_(SM).In the location of ¼ toward the center in the sheet thickness directionfrom the steel sheet surface, EDX analysis was performed on parts otherthan the white and light gray parts in each of ten fields of view, theaverage Mn content (Mn content in martensite) was calculated, and[Mn]_(B) was determined from the fraction of martensite and the Mncontent in martensite and from the fractions of phases other thanmartensite and the Mn contents in phases other than martensite.

<Steel Microstructure of Steel Sheet and Plated Steel Sheet>

Area Ratio of Polygonal Ferrite within Range of 20 μm in Sheet ThicknessDirection from Steel Sheet Surface: 0 to 80%

When the polygonal ferrite is formed within the range of 20 μm in thesheet thickness direction from the steel sheet surface, bendabilitydeteriorates due to the difference in hardness between the ferrite andmartensite. For this reason, the area ratio of polygonal ferrite needsto be reduced as much as possible. In the steel sheet of the disclosedembodiments, the area ratio of polygonal ferrite can be 80% or less.Accordingly, the area ratio of polygonal ferrite within a range of 20 μmin the sheet thickness direction from the steel sheet surface is 0 to80%, preferably less than 40%, more preferably 38% or less.

Area Ratio of Martensite, Bainite, and Residual Austenite within Rangeof 20 μm in Sheet Thickness Direction from Steel Sheet Surface: 20 to100% in Total

Excellent bendability can be obtained in the steel sheet and the platedsteel sheet of the disclosed embodiments by formation of a large amountof martensite, bainite, and residual austenite microstructure within therange of 20 μm in the sheet thickness direction from the steel sheetsurface. For this reason, the area ratio of martensite, bainite, andresidual austenite within the range of 20 μm in the sheet thicknessdirection from the steel sheet surface is 20 to 100% in total,preferably 50% or more. The area ratio of them is 98% or less, morepreferably 90% or less. Since bendability can be further improved whenthe area ratio of martensite in the microstructure is 15 to 40%, thearea ratio of martensite is preferably 15 to 40%.

Preferably, the area ratio of polygonal ferrite and martensite withinthe range of 20 μm in the sheet thickness direction from the steel sheetsurface is 86% or less in total, more preferably 75% or less.Bendability is further improved when the area ratio of polygonal ferriteand martensite is 86% or less in total.

Area Ratio of Martensite at Location of 300 μm in Sheet ThicknessDirection from Steel Sheet Surface: 20 to 50%

TS of 980 MPa or more can hardly be obtained when the area ratio ofmartensite in the vicinity of 300 μm in the sheet thickness directionfrom the steel sheet surface is less than 20%. On the other hand, whenthe area ratio of martensite exceeds 50%, bendability deteriorates. Forthis reason, the area ratio of martensite in the vicinity of 300 μm inthe sheet thickness direction from the steel sheet surface is preferably20 to 50%. Preferably, the area ratio of martensite is 25% or more.Preferably, the area ratio of martensite is 45% or less. Morepreferably, the area ratio of martensite is less than 40%, further morepreferably 38% or less.

Basically, pearlite is not contained in the steel sheet, but whencontained, the area ratio is preferably 3% or less.

As used herein, the area ratio of polygonal ferrite, martensite,bainite, and residual austenite means the area proportion of eachmicrostructure in the observed area, and is measured in the mannerdescribed below. A sample is cut from the annealed steel sheet, and across section taken through the thickness parallel to the rollingdirection is polished. Then, after corroding the surface with 3% nital,a micrograph is taken for three fields of view at a location in thevicinity of the steel sheet surface (a location of 20 m in the sheetthickness direction from the steel sheet surface) and at a location of300 m in the sheet thickness direction from the steel sheet surface,using a SEM (scanning electron microscope) at a magnification of 1,500times. The area ratio of each microstructure is determined from theobtained image data, using the Image-Pro available from MediaCybernetics, and the average area ratio of these fields of view isdefined as the area ratio of each microstructure. In the image data, thepolygonal ferrite appears black having a smooth curved grain boundary,martensite and residual austenite appear white or light gray, andbainite appears gray or dark gray including carbides having a straightgrain boundary with aligned orientations or island-like martensite,whereby these phases are distinguished from each other. According to themethod of the disclosed embodiments, tempered martensite having darkgray or black color and including carbides is not formed, but suchtempered martensite may deteriorate bendability, so it is preferably notcontained. Martensite including carbides with non-aligned orientationsdiffers from bainite. In the steel sheet of the disclosed embodiments,the carbides can be distinguished as a white dot or a line. Although notcontained in the steel sheet of the disclosed embodiments, pearlite canbe distinguished as a laminar structure of black and white.

<Steel Sheet>

The composition and the steel microstructure of the steel sheet are asdescribed above. In addition, the thickness of the steel sheet is notparticularly limited, and is typically 0.4 mm or more and 6.0 mm orless.

<Plated Steel Sheet>

The plated steel sheet of the disclosed embodiments is a plated steelsheet including a plating layer on a surface of the steel sheet of thedisclosed embodiments. The plating layer is not particularly limited,and may be, for example, a hot-dip plating layer or an electroplatingplating layer. Further, the plating layer may also be an alloyed platinglayer. The plating layer is preferably a galvanized layer. Thegalvanized layer may contain Al or Mg. Hot-dip zinc-aluminum-magnesiumalloy plating (Zn—Al—Mg plating layer) is also preferred. In this case,the Al content is preferably 1 mass % or more and 22 mass % or less, andthe Mg content is preferably 0.1 mass % or more and 10 mass % or less.The Zn—Al—Mg plating layer also may contain one or more selected fromSi, Ni, Ce, and La in a total amount of 1 mass % or less. The platedmetal is not particularly limited, and metals such as aluminum may beplated, other than zinc described above.

The composition of the plating layer is not particularly limited, andthe plating layer may have a common composition. The plating layer maybe, for example, a hot-dip galvanized layer or a hot-dip galvannealedlayer containing Fe: 20.0 mass % or less, Al: 0.001 mass % or more and1.0 mass % or less, one or more selected from Pb, Sb, Si, Sn, Mg, Mn,Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% or more and 3.5 mass % or less, and the balance being Zn andunavoidable impurities. For example, the plating layer may preferably bea hot-dip galvanized layer with the plating metal in an amount of 20 to80 g/m² for each side, or a hot-dip galvannealed layer produced as analloyed layer of such plating layers. When the plating layer is ahot-dip galvanized layer, the Fe content in the plating layer is lessthan 7 mass %. When the plating layer is a hot-dip galvannealed layer,the Fe content in the plating layer is 7 to 15 mass %.

Production methods of the disclosed embodiments will be described below.

<Method of Production of Hot-Rolled Steel Sheet>

A method for producing a hot-rolled steel sheet of the disclosedembodiments uses a slab of the composition described above for thehot-rolled steel sheet, the cold-rolled full hard steel sheet, the steelsheet, and the plated steel sheet, and includes: hot rolling the slab,the hot rolling including finish-rolling under conditions that atemperature from a second-last pass to the last pass is 800 to 950° C.,a cumulative rolling reduction from the second-last pass to the lastpass is 10 to 40%, and a rolling reduction of the last pass is 8 to 25%;cooling with start in a range of 0.5 to 3.0 sec after an end offinish-rolling, at an average cooling rate of 30° C./s or more in atemperature range of 600 to 720° C.; and coiling at a temperature of590° C. or lower. The following describes these conditions. In thefollowing, the temperature means steel sheet surface temperature, unlessotherwise specifically stated. The steel sheet surface temperature maybe measured with, for example, a radiation thermometer. The averagecooling rate is represented by ((surface temperature beforecooling−surface temperature after cooling)/cooling time).

Production of Slab

The melting method for production of the slab is not particularlylimited, and various known melting methods may be used, including, forexample, a method using a converter furnace and a method using anelectric furnace. It is also possible to perform secondary refining witha vacuum degassing furnace. After that, preferably, the slab (steelmaterial) may be produced by a known continuous casting method for sakeof prevention of macro segregation. Further, the slab may be producedusing known casting methods such as ingot casting-slabbing rolling andthin-slab continuous casting.

For hot rolling of a slab, the slab may be first cooled to roomtemperature, and then reheated for hot rolling, or the slab may be hotrolled by being charged into a heating furnace without being cooled toroom temperature. Alternatively, an energy-saving process may be appliedin which hot rolling is performed immediately after slightly warming theslab.

When heating the slab, the slab is heated to preferably 1,100° C. orhigher to dissolve the carbides, or to prevent increase of the rollingload. The slab heating temperature is preferably 1,300° C. or lower toprevent increase of a scale loss. The slab temperature is thetemperature of a slab surface.

The next step is hot rolling. The rough rolling conditions are notparticularly limited. The sheet bar after rough rolling may be heated.It is also possible to perform continuous rolling, in which finishrolling is continuously performed on joined sheet bars.

Temperature from the Second-last Pass to Last pass: 800 to 950° C.

In the disclosed embodiments, it is important, from the viewpoint ofhot-rolled microstructure and annealed microstructure formation, todefine a cumulative rolling reduction and a temperature from thesecond-last pass from the last pass in the finish rolling.

When the finish rolling temperature is lower than 800° C., ferrite isformed, and unevenness in concentration of Mn occurs in the surfacelayer of the hot-rolled steel sheet to cause enrichment of Mn inaustenite during annealing, whereby [Mn]_(SM)/[Mn]_(B) exceeds 1.5 andbendability and plating capabilities deteriorate. On the other hand, thefinish rolling temperature is higher than 950° C., coarse grains areformed on the surface layer of the hot-rolled steel sheet to causecoarse polygonal ferrite during subsequent annealing, whereby Mn isenriched in austenite, [Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendabilityand plating capabilities deteriorate. For this reason, the temperaturefrom the second-last pass to the last pass is 800 to 950° C. Preferably,the temperature is 830° C. or higher. The temperature is preferably 920°C. or lower.

Cumulative Rolling Reduction from the Second-Last Pass to Last Pass: 10to 40%

When the cumulative rolling reduction of the second-last pass to thelast pass is less than 10%, worked austenite remains to promote ferriteformation and to cause unevenness in concentration of Mn in the surfacelayer of the hot-rolled steel sheet, Mn is enriched in austenite duringannealing, [Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendability and platingcapabilities deteriorate. On the other hand, when the cumulative rollingreduction of the second-last pass to the last pass exceeds 40%,recrystallization is excessively promoted, a coarse microstructureremains on the surface layer of the hot-rolled steel sheet, coarsepolygonal ferrite is formed during subsequent annealing to causeenrichment of Mn in austenite, [Mn]_(SN)/[Mn]_(B) exceeds 1.5, andbendability and plating capabilities deteriorate.

Rolling Reduction of Last Pass: 8 to 25%

When the rolling reduction of the last pass is less than 8%, expandedgrains remain, coarse polygonal ferrite is formed during annealing tocause enrichment of Mn in austenite, [Mn]_(SM)/[Mn]_(B) exceeds 1.5, andbendability and plating capabilities deteriorate. On the other hand,when the rolling reduction of the last pass exceeds 25%, ferriteformation is promoted to cause unevenness in concentration of Mn in thesurface layer of the hot-rolled steel sheet, Mn is enriched in austeniteduring annealing, [Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendability andplating capabilities deteriorate. For this reason, the rolling reductionof the last pass is 8 to 25%. Preferably, the rolling reduction of thelast pass is 10% or more and 20% or less.

Cooling with Start in a Range of 0.5 to 3.0 Sec after End of FinishRolling

When the time until the start of cooling from the end of finish rollingis less than 0.5 s, large strain is accumulated in austenite, resultingin promoting ferrite formation, whereby unevenness in concentration ofMn is caused in the surface layer of the hot-rolled steel sheet, Mn isenriched in austenite during annealing, [Mn]_(SM)/[Mn]_(B) exceeds 1.5,and bendability and plating capabilities deteriorate. On the other hand,the time exceeds 3.0 sec, the strain in austenite is completelyreleased, a coarse microstructure remains on the surface layer of thehot-rolled steel sheet, coarse polygonal ferrite is formed duringsubsequent annealing to cause enrichment of Mn in austenite,[Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendability and plating capabilitiesdeteriorate. For this reason, the time until the start of cooling fromthe finish rolling is 0.5 to 3.0 sec.

Cooling at Average Cooling Rate of 30° C./s or More in Temperature Rangeof 600 to 720° C.

When the average cooling rate in the temperature range of 600 to 720° C.is less than 30° C./s, ferrite is formed to cause unevenness inconcentration of Mn in the surface layer of the hot-rolled steel sheetand to cause enrichment of Mn in austenite during annealing,[Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendability and plating capabilitiesdeteriorate. For this reason, the average cooling rate in thetemperature range of 600 to 720° C. is 30° C./s or more. Although notparticularly specified, the upper limit of the average cooling rate ispreferably 1,000° C./s or less. This is because characteristic variationis caused due to unevenness in temperature when the average cooling rateexceeds 1000° C./s.

In order to reduce the rolling load, or to make uniform shapes ormaterials, it is preferable to perform lubrication rolling that makesthe coefficient of friction 0.10 to 0.25 in some of or all of the passesof the finish rolling.

Coiling Temperature: 590° C. or Lower

When the coiling temperature is higher than 590° C., ferrite formationis promoted to cause unevenness in concentration of Mn in the surfacelayer of the hot-rolled steel sheet, Mn is enriched in austenite duringannealing, [Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendability and platingcapabilities deteriorate. For this reason, the coiling temperature is590° C. or lower. The coiling temperature is preferably higher than 300°C. in terms of bendability.

The steel sheet is cooled by air cooling or the like after the coiling,and is used for production of a cold-rolled full hard steel sheet asdescribed below. When the hot-rolled steel sheet is to be sold in theform of an intermediate product, the hot-rolled steel sheet is typicallyprepared into a commercial product after being coiled and cooled.

<Method of Production of Cold-Rolled Full Hard Steel Sheet>

A method for producing a cold-rolled full hard steel sheet of thedisclosed embodiments is a method in which the hot-rolled steel sheetobtained by the above-described method is cold rolled at a rolling ratioof 20% or more.

In the disclosed embodiments, it is necessary to make the rollingreduction 20% or more when the steel sheet is subjected to cold rolling.When the rolling reduction is less than 20%, coarse ferrite is formedduring annealing to cause enrichment of Mn in austenite,[Mn]_(SM)/[Mn]_(B) exceeds 1.5, and bendability and plating capabilitiesdeteriorate. For this reason, when the cold rolling is performed, therolling reduction is 20% or more, preferably 30% or more. Although notbe particularly specified, the upper limit of the rolling reduction ispreferably 95% or less in terms of shape stability or the like.

Pickling may be performed before the cold rolling. The picklingconditions may be appropriately set.

<Method of Production of Steel Sheet>

A method for producing a steel sheet of the disclosed embodiments is amethod that includes: heating the hot-rolled steel sheet or thecold-rolled full hard steel sheet obtained by the above-describedproduction method up to a temperature of 730 to 900° C.; cooling thesteel sheet to the cooling stop temperature of 400 to 590° C. at theaverage cooling rate of 5 to 50° C./s; retaining the steel sheet for 10to 1000 sec in the temperature range of 730 to 900° C. from the heatingto the cooling; and annealing it under a retention of 1000 sec or lessin the temperature range of 400 to 590° C. The annealing may be followedby temper rolling, as required.

Heating to Temperature of 730 to 900° C. (Annealing Temperature)

When the annealing temperature is lower than 730° C., austenite isinsufficiently formed. Since austenite formed by annealing becomesmartensite or bainite in the final microstructure due to bainitetransformation or martensite transformation, when austenite isinsufficiently formed, the microstructure of the steel sheet of thedisclosed embodiments cannot be obtained. On the other hand, when theannealing temperature is higher than 900° C., surface enrichment of Siand Mn becomes large, resulting in non-plating. For this reason, theannealing temperature is 730 to 900° C. Preferably, the annealingtemperature is 740° C. or higher. The annealing temperature ispreferably 860° C. or lower.

Average Cooling Rate from Annealing Temperature to Cooling StopTemperature of 400 to 590° C.: 5 to 50° C./s

When the average cooling rate is less than 5° C., polygonal ferrite isexcessively formed, and the microstructure of the steel sheet of thedisclosed embodiments cannot be obtained. On the other hand, when theaverage cooling rate is more than 50° C./s, bainite transformation ispromoted, and the microstructure of the steel sheet of the disclosedembodiments cannot be obtained. For this reason, the average coolingrate from the annealing temperature to the cooling stop temperature of400 to 590° C. is 5 to 50° C./s. Preferably, the average cooling rate is8° C./s or more and 30° C./s or less.

When the cooling stop temperature is lower than 400° C., temperedmartensite is formed and bendability deteriorates. On the other hand,when the cooling stop temperature is higher than 590° C., polygonalferrite is excessively formed, and the microstructure of the steel sheetof the disclosed embodiments cannot be obtained. For this reason, thecooling stop temperature is 400 to 590° C. Preferably, the cooling stoptemperature is 440° C. or higher and 560° C. or lower.

Retention for 10 to 1000 Sec in Temperature Range of 730 to 900° C. ofHeating to Cooling

When the retention time in the temperature range of 730 to 900° C. isless than 10 sec, austenite is insufficiently formed, and themicrostructure of the steel sheet of the disclosed embodiments cannot beobtained. On the other hand, when the retention time is more than 1000sec, Mn is enriched in austenite, [Mn]_(SM)/[Mn]_(B) exceeds 1.5, andbendability and plating capabilities deteriorate. For this reason, theretention time is 10 to 1000 sec. Preferably, the retention time is 30sec or more. The retention time is preferably 500 sec or less. Here, theretention time is the dwell time (pass time) of the steel sheet in theannealing temperature range described above. The retention does notnecessarily represent a constant state, and includes heating and coolingstates in the temperature range of 730 to 900° C.

Retention for 1000 Sec or Less in Temperature Range of 400 to 590° C.

When the retention time in the temperature range of 400 to 590° C.exceeds 1000 sec, ferrite transformation and bainite transformationexcessively proceed or pearlite is formed, whereby the microstructure ofthe steel sheet of the disclosed embodiments cannot be obtained. Forthis reason, the retention time in the temperature range of 400 to 590°C. is 1000 sec or less, preferably 500 sec or less, more preferably 200sec or less. Here, the retention time is the dwell time (pass time) ofthe steel sheet in the temperature range described above. The retentiondoes not necessarily represent a constant state.

Dew Point in Temperature Range of 730 to 900° C.: −40° C. or Lower(Preferred Condition)

By making the dew point −40° C. or lower in the temperature range of 730to 900° C., enrichment of Si and Mn onto the steel sheet surface can bereduced, [Mn]_(SM)/[Mn]_(B) can be reduced, and bendability and platingcapabilities can be further improved. For this reason, the dew point inthe temperature range of 730 to 900° C. is preferably −40° C. or lower,more preferably −45° C. or lower. The lower limit of the dew point ofthe atmosphere is not particularly specified. However, the dew point ispreferably −80° C. or higher because the effect becomes saturated whenthe dew point is lower than −80° C., and poses cost disadvantages. Thetemperature in the above-described temperature range is based on thesurface temperature of the steel sheet. Specifically, the dew point isadjusted in the above-described range when the surface temperature ofthe steel sheet is in the above-described temperature range.

Elongation Ratio of Temper Rolling: 0.6% or Less (Preferred Condition)

Temper rolling is performed after the cooling, as needed. The temperrolling introduces dislocation, and the aging resistance becomes poor.For this reason, the elongation ratio of temper rolling is preferably0.6% or less. From the viewpoint of sheet surface qualities or sheetshape, the elongation ratio of temper rolling is preferably 0.1% ormore.

When the steel sheet is to be sold, the steel sheet is typically soldafter being cooled to room temperature following the cooling or temperrolling.

As described above, according to the disclosed embodiments, a desiredsteel sheet can be obtained only by primary annealing. After the primaryannealing, secondary annealing may be performed under normal conditions.However, only the primary annealing is preferably performed inconsideration of costs without the secondary annealing.

<Method of Production of Plated Steel Sheet>

A method for producing a plated steel sheet of the disclosed embodimentsis a method by which the steel sheet obtained above is plated. Platingmay be, for example, a hot-dip galvanizing process, or a process thatinvolves alloying after hot-dip galvanizing. Annealing and galvanizingmay be continuously performed on the same line. The plating layer may beformed by electroplating such as electroplating of a Zn—Ni alloy, or maybe formed by hot-dip plating of a zinc-aluminum-magnesium alloy.Preferred is galvanizing, as described above in conjunction with theplating layer. It is, however, possible to perform plating using othermetals such as aluminum. The following descriptions are given throughthe case of hot-dip plating.

Hot-dip plating is performed by dipping the steel sheet in a platingbath. In the case of this method, the steel sheet (steel sheet) dippedin a plating bath needs to be adjusted to a temperature of 450° C. orhigher and 550° C. or lower. With temperatures outside of thetemperature range of the 450° C. or higher and 550° C. or lower, foreignsubstance may occur in the plating bath, or it may not be possible tocontrol the plating bath temperature. For this reason, the temperatureof the steel sheet dipped in the plating bath is adjusted to 450° C. orhigher and 550° C. or lower, preferably 460° C. or higher and 540° C. orlower.

The hot-dip plating may be followed by an alloying treatment, asrequired. The alloying treatment temperature, and the alloying treatmenttime are not particularly limited, and may be appropriately set.

After the steel sheet is produced by a continuous hot-dip plating line,and the plated steel sheet may be immediately produced using the steelsheet.

EXAMPLES Example 1

The disclosed embodiments will be described in detail below withreference to Examples. The technical scope of the disclosed embodimentsis not limited to the following Examples.

Steels of the compositions (with the balance Fe and unavoidableimpurities) shown in Table 1 were melted with a vacuum melting furnace,and prepared into a steel slab by rolling. The steel slabs were heatedto 1, 200° C., rough rolled, and subjected to hot rolling under theconditions shown in Table 2 to produce hot-rolled steel sheets (HR).Some of the steel sheets were cold rolled to a thickness of 1.4 mm toobtain cold-rolled full hard steel sheets (CR). The hot-rolled steelsheets and the cold-rolled full hard steel sheets were annealed. Thesteel sheets were then subjected to hot-dip galvanizing, and, asrequired, hot-dip galvannealing to produce hot-dip galvanized steelsheets (GI), and hot-dip galvannealed steel sheets (GA). The annealingwas performed in a plating treatment apparatus under the conditionsshown in Table 2. The hot-dip galvanized steel sheets were dipped in a465° C. plating bath to form a plating layer with a coating weight of 35to 45 g/m² for each side. The hot-dip galvannealed steel sheets weresubjected to alloying in which the plated steel sheets were retained at500 to 600° C. for 1 to 60 sec to make the Fe content in the platinglayer 6 mass % or more and 15 mass % or less. The plated steel sheetswere cooled at 8° C./s. The hot-dip galvanized steel sheets, and thehot-dip galvannealed steel sheets were then subjected to temper rollingat an elongation ratio of 0.3%, and tested to evaluate the tensilecharacteristics, bendability, and plating capabilities in the mannerdescribed below. The steel sheets were also measured for microstructure.The results of these measurements are presented in Table 3.

<Observation of Microstructure>

The area ratio of each phase was evaluated using the followingtechnique. A sample is cut from the a steel sheet such that a crosssection parallel to the rolling direction is an observation surface, andthe cross section is polished. Then, after corroding the surface with 3%nital, a micrograph is taken for three fields of view at a location inthe vicinity of the steel sheet surface (a location of 20 μm in thesheet thickness direction from the steel sheet surface) and at alocation of 300 μm in the sheet thickness direction from the steel sheetsurface, using a SEM (scanning electron microscope) at a magnificationof 1,500 times. The area ratio of each microstructure is determined fromthe obtained image data, using the Image-Pro available from MediaCybernetics, and the average area ratio of these fields of view isdefined as the area ratio of each microstructure. A micrograph is takenfor ten fields of view at a ¼-thickness of the sheet. In the image data,the polygonal ferrite appears black having a smooth curved grainboundary, the martensite and residual austenite appear white or lightgray, and bainite appears gray or dark gray including carbides having astraight grain boundary with aligned orientations or island-likemartensite, whereby these phases are distinguished from each other. Inthe disclosed embodiments, martensite includes auto-tempered martensitecontaining carbides.

<Tensile Test>

A JIS 5 tensile test piece (JIS Z 2201) was collected from the hot-dipgalvanized steel sheet (GI) or the hot-dip galvannealed steel sheet (GA)in a direction orthogonal to the rolling direction, and subjected to aJIS Z 2241 tensile test at a strain rate of 10⁻³/s to determine TS andEL. In the disclosed embodiments, samples were acceptable when the TSwas 980 MPa or more.

<Bendability>

A strip-shaped test piece having 30 mm in width and 100 mm in length wascollected from the hot-dip galvanized steel sheet (GI) or the hot-dipgalvannealed steel sheet (GA), and subjected to a flexure test in whichthe test piece was bent in a direction parallel to the rollingdirection. By performing a V-bend test at an angle of 90° under theconditions of a stroke speed of 500 mm/s, a press load of 10 ton, and apress-retention time of 5 sec, and by observing the ridge line at thebending position using a magnifier at a magnification of 10 times, theminimum bending radius (mm) at which cracks of 0.5 mm or more could notrecognized was determined, and R/t was calculated by dividing theminimum bending radius by the sheet thickness (mm). Samples wereacceptable when R/t was 2.5 or less.

<Plating Capabilities>

A strip-shaped test piece having 30 mm in width and 30 mm in length wascollected from the hot-dip galvanized steel sheet or the hot-dipgalvannealed steel sheet, and the surface of the steel sheet wasobserved using a magnifier at a magnification of 10 times. Samples wereacceptable when non-plating having a diameter of 0.5 mm or more wasrecognized.

TABLE 1 Composition (mass %) Steel C Si Mn P S Al N Ti Nb Mo OtherRemarks A 0.10 0.10 2.3 0.015 0.001 0.032 0.005 0.025 0.015 — — ExampleB 0.08 0.20 2.6 0.024 0.002 0.031 0.003 0.012 0.012 0.050 — Example C0.13 0.55 1.9 0.010 0.003 0.028 0.002 — 0.035 0.100 Cr: 0.500 Example D0.10 0.25 2.5 0.018 0.005 0.041 0.008 0.033 — 0.080 Ni: 0.500 Example E0.11 0.05 2.8 0.007 0.001 0.033 0.004 — 0.055 — B: 0.0012 Example F 0.100.01 2.2 0.003 0.002 0.035 0.003 0.021 — — Cu: 0.200, Ca: 0.0010 ExampleG 0.17 0.10 2.4 0.030 0.002 0.021 0.004 — — 0.150 V: 0.100, Sb: 0.0100Example H 0.14 0.20 2.5 0.021 0.003 0.038 0.003 0.015 0.017 0.020 Sb:0.0100 Example I 0.12 0.40 2.1 0.017 0.002 0.018 0.007 0.038 0.029 — Sn:0.0500, REM: 0.0020 Example J 0.04 0.05 2.2 0.020 0.001 0.044 0.0030.018 0.130 — Comparative Example K 0.28 0.05 2.5 0.022 0.001 0.0370.003 0.013 0.024 0.110 — Comparative Example L 0.16 1.10 1.8 0.0140.002 0.025 0.004 — 0.043 0.080 — Comparative Example M 0.16 0.20 1.40.019 0.002 0.034 0.004 — — 0.220 — Comparative Example N 0.16 0.15 4.10.015 0.003 0.035 0.003 0.020 — 0.100 — Comparative Example O 0.16 0.012.6 0.025 0.002 0.029 0.003 — — — — Comparative Example P 0.16 0.01 2.60.025 0.002 0.049 0.003 0.120 — — — Comparative Example

TABLE 2 Hot rolling conditions Annealing Cooling Retention Cumulativestart Cold time in Temperature rolling Last time Average rollingtemperature from second reduction pass after cooling rate Cold range ofSteel pass to last All rolling from second rolling finish- betweenCoiling rolling Annealing 730° C. to sheet pass reduction pass to lastreduction rolling 600 and 720° C. temperature reduction temperature 900°C. No. Steel (° C.) (%) pass (%) (s) (° C./s) (° C.) (%) (° C.) (s) 1 A890 85 30 15 1.0 200 500 50 810 200 2 990 85 30 15 1.0 200 500 50 810200 3 750 85 30 15 1.0 200 500 50 810 200 4 890 85 35 30 1.0 200 500 50810 200 5 890 85 30 5 1.0 200 500 50 810 200 6 B 890 85 30 15 1.0 50 50050 810 200 7 890 85 30 15 1.0 20 500 50 810 200 8 890 85 30 15 1.0 200620 50 810 200 9 890 85 30 15 1.0 200 500 15 810 200 10 C 890 85 20 151.0 200 450 40 810 200 11 890 85 30 15 1.0 200 450 40 700 200 12 890 8530 15 1.0 200 450 40 920 200 13 890 85 30 15 1.0 200 450 40 810 1 14 89085 30 15 1.0 200 450 50 810 1200 15 D 890 85 30 15 1.0 200 500 — 810 20016 890 85 30 15 1.0 200 500 — 810 200 17 890 85 30 15 1.0 200 500 — 810200 18 890 85 30 15 1.0 200 500 — 810 200 19 890 85 30 15 1.0 200 500 —810 200 20 E 890 85 30 15 1.0 200 500 50 790 200 21 890 85 30 15 1.0 200500 50 810 200 22 F 890 85 25 15 1.0 200 500 50 850 200 23 G 890 85 3015 1.0 200 500 50 750 200 24 890 85 30 15 0.1 200 500 50 750 200 25 89085 30 15 5.0 200 500 50 750 200 26 H 890 85 30 15 1.0 200 500 50 810 20027 890 85 30 15 1.0 200 500 50 810 200 28 I 890 85 30 15 1.0 200 500 50810 200 29 890 85 9 5 2.0 200 500 50 810 201 30 890 85 42 15 3.0 200 50050 810 202 31 J 890 85 30 15 1.0 200 500 50 810 200 32 K 890 85 30 151.0 200 500 50 810 200 33 L 890 85 30 15 1.0 200 500 50 810 200 34 M 89085 30 15 1.0 200 500 50 810 200 35 N 890 85 30 15 1.0 200 500 50 810 20036 O 890 85 30 15 1.0 200 500 50 810 200 37 P 890 85 30 15 1.0 200 50050 810 200 38 A 890 85 30 15 1.0 200 500 50 810 200 39 A 890 85 30 151.0 200 500 50 810 200 Annealing Plating Dew point Average CoolingPlating Steel between 730° C. cooling stop Cooling stop bath AlloyingAlloying sheet and 900° C. rate temperature retention time temperaturetemperature retention No. (° C.) (° C./s) (° C.) (s) (° C.) (° C.)time(s) *Plating condition Remarks 1 −45 15 500 60 465 520 20 GA PresentExample 2 −45 15 500 60 465 520 20 GA Comparative Example 3 −45 15 50060 465 520 20 GA Comparative Example 4 −45 15 500 60 465 520 20 GAComparative Example 5 −45 15 500 60 465 520 20 GA Comparative Example 6−49 30 500 100 465 — — GI Present Example 7 −49 30 500 100 465 — — GIComparative Example 8 −49 30 500 100 465 — — GI Comparative Example 9−49 30 500 100 465 — — GI Comparative Example 10 −55 8 500 15 465 530 30GA Present Example 11 −55 8 500 15 465 530 30 GA Comparative Example 12−55 8 500 15 465 530 30 GA Comparative Example 13 −55 8 500 15 465 53030 GA Comparative Example 14 −55 8 500 15 465 530 30 GA ComparativeExample 15 −47 15 500 100 465 520 20 GA Present Example 16 −47 15 620100 465 520 20 GA Comparative Example 17 −47 15 350 100 465 520 20 GAComparative Example 18 −47 1 500 100 465 520 20 GA Comparative Example19 −47 75 500 100 465 520 20 GA Comparative Example 20 −47 15 460 800465 520 20 GA Present Example 21 −47 15 460 1200 465 520 20 GAComparative Example 22 −47 15 550 100 465 — — GI Present Example 23 −4715 500 100 465 520 20 GA Present Example 24 −47 15 500 100 465 520 20 GAComparative Example 25 −47 15 500 100 465 520 20 GA Comparative Example26 −47 15 500 100 465 520 20 GA Present Example 27 −38 15 500 100 465520 20 GA Present Example 28 −47 15 500 100 465 520 20 GA PresentExample 29 −47 15 501 100 465 520 20 GA Comparative Example 30 −47 15502 100 465 520 20 GA Comparative Example 31 −47 15 500 100 465 520 20GA Comparative Example 32 −47 15 500 100 465 520 20 GA ComparativeExample 33 −47 15 500 100 465 550 20 GA Comparative Example 34 −47 15500 100 465 520 20 GA Comparative Example 35 −47 15 500 100 465 520 20GA Comparative Example 36 −47 15 500 100 465 520 20 GA ComparativeExample 37 −47 15 500 100 465 520 20 GA Comparative Example 38 −47 15600 100 465 520 20 GA Comparative Example 39 −47 15 500 100 465 520 20GA Present Example *Plating condition: GI: hot-dip galvanized steelsheet, GA: hot-dip galvannealed steel sheet

TABLE 3 Mechanical *Microstructure characteristics Steel Region fromsurface to 20 μm Position TS Plating sheet No. V (PF) (%) V (M + B + γ)(%) V (M) (%) Other (%) V (M) (%) [Mn]_(SM)/[Mn]_(s) (MPa) R/tcapability Remarks 1 38 62 28 0 32 1.1 1017 0.7 ◯ Present Example 2 4456 25 0 29 1.7 993 3.2 X Comparative Example 3 37 63 26 0 30 1.8 10003.2 X Comparative Example 4 34 66 27 0 33 1.6 1020 2.9 X ComparativeExample 5 36 64 28 0 31 1.7 1004 3.2 X Comparative Example 6 18 82 31 038 1.1 1035 1.4 ◯ Present Example 7 19 81 33 0 37 1.8 1033 3.2 XComparative Example 8 20 80 35 0 39 1.8 1040 2.9 X Comparative Example 920 80 34 0 37 1.7 1256 3.2 X Comparative Example 10 32 68 41 0 43 1.31175 2.1 ◯ Present Example 11 97 3 1 0 2 2.0 714 0.7 X ComparativeExample 12 0 100 25 0 36 1.6 1135 2.9 X Comparative Example 13 82 18 160 16 1.5 897 2.5 ◯ Comparative Example 14 35 65 40 0 42 1.7 1067 2.9 XComparative Example 15 43 57 22 0 25 1.1 986 1.1 ◯ Present Example 16 8515 15 0 28 1.1 982 2.9 ◯ Comparative Example 17 33 31 11 0 14 1.1 10593.2 ◯ Comparative Example 18 88 12 11 0 12 1.3 757 0.7 ◯ ComparativeExample 19 24 76 18 0 19 1.2 969 1.4 ◯ Comparative Example 20 55 45 31 032 1.2 1015 1.7 ◯ Present Example 21 41 56 16 3 16 1.1 908 1.1 ◯Comparative Example 22 2 98 33 0 31 1.0 985 0.7 ◯ Present Example 23 5842 36 0 35 1.4 1071 2.5 ◯ Present Example 24 55 45 38 0 36 1.8 1085 3.2X Comparative Example 25 57 43 35 0 35 1.7 1069 2.9 X ComparativeExample 26 25 75 32 0 33 1.2 1024 1.4 ◯ Present Example 27 29 71 36 0 371.5 1060 1.7 ◯ Present Example 28 68 32 31 0 33 1.4 1032 2.5 ◯ PresentExample 29 70 30 28 0 31 1.6 1025 3.2 X Comparative Example 30 68 32 300 31 1.7 1035 3.2 X Comparative Example 31 66 34 2 0 5 12 839 0.4 ◯Comparative Example 32 7 93 42 0 45 1.0 1193 3.9 ◯ Comparative Example33 52 48 39 0 37 1.3 939 1.4 X Comparative Example 34 83 17 10 0 12 1.3778 1.1 ◯ Comparative Example 35 0 100 100 0 100 1.0 1426 3.9 ◯Comparative Example 36 5 95 33 0 32 1.0 1020 3.2 ◯ Comparative Example37 16 84 29 0 30 1.0 1018 2.9 ◯ Comparative Example 38 66 18 18 16 181.1 886 0.4 ◯ Comparative Example 39 32 68 45 0 45 1.1 1069 2.5 ◯Present Example *V (PF): area ratio of polygonal ferrite, V (M + B + γ):total area ratio of martensite and bainite and retained austenite, V(M): area ratio of martensite Other: area ratio of phases except for theabove ones

Present examples are all high-strength galvanized steel sheets having TSof 980 MPa or more and R/t of 2.5 or less with no-plating. ComparativeExamples outside of the ranges of the disclosed embodiments had at leastone of TS, R/t, and plating capabilities inferior to those of thepresent examples.

No. 27 is a present example in which the dew point is out of thepreferred range. [Mn]_(SM)/[Mn]_(B) deteriorated as compared with otherpresent examples in which the dew point was within the preferred range,and bendability and plating capabilities were slightly inferior but hadno problem in terms of an effect.

The invention claimed is:
 1. A steel sheet having a chemical compositioncomprising, by mass: C: more than 0.06 to 0.25%; Si: 1.0% or less; Mn:1.5 to 4.0%; P: 0.100% or less; S: 0.02% or less; Al: 1.0% or less; N:0.001 to 0.015%; at least one element selected from the group consistingof Ti: 0.003 to 0.100%, Nb: 0.003 to 0.100%, and Mo: 0.005 to 0.500%;and a balance being Fe and unavoidable impurities, wherein: the steelsheet has a microstructure comprising, in terms of an area ratio: in aregion of the steel sheet that extends from a surface of the steel sheetto a position 20 μm from the surface of the steel sheet in a sheetthickness direction: polygonal ferrite in a range of 0 to 80%, and atotal of martensite, bainite, and residual austenite in a range of 20 to100%, and at a location 300 μm from the surface of the steel sheet inthe sheet thickness direction, martensite in a range of 20 to 50%, and[Mn]_(SM)/[Mn]_(B)≤1.5, where: [Mn]_(SM) represents a content of Mn inmartensite present in the region of the steel sheet that extends fromthe surface of the steel sheet to the position 20 μm from the surface ofthe steel sheet, and [Mn]_(B) represents a content of Mn at a positionlocated a distance of ¼-thickness of the steel sheet toward a center ofthe steel sheet in the sheet thickness direction from the surface of thesteel sheet.
 2. The steel sheet according to claim 1, wherein thechemical composition further comprises, by mass %, at least one elementselected from the group consisting of: Cr: 0.005 to 2.000%, V: 0.005 to2.000%, Cu: 0.005 to 2.000%, Ni: 0.005 to 2.000%, B: 0.0001 to 0.0050%,Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.1000%,and Sn: 0.0010 to 0.5000%.
 3. The steel sheet according to claim 1,wherein the chemical composition comprises, by mass %, C in an amount ina range of from 0.07% to 0.25%.
 4. A plated steel sheet comprising aplating layer disposed on the surface of the steel sheet of claim
 1. 5.A plated steel sheet comprising a plating layer disposed on the surfaceof the steel sheet of claim
 2. 6. The plated steel sheet according toclaim 4, wherein the plating layer is a hot-dip galvanized layer or ahot-dip galvannealed layer.
 7. The plated steel sheet according to claim5, wherein the plating layer is a hot-dip galvanized layer or a hot-dipgalvannealed layer.
 8. A method of producing the steel sheet of claim 1,the method comprising: hot rolling a slab having the chemicalcomposition to produce a hot-rolled steel sheet, the hot rollingcomprising finish rolling under conditions in which a temperature from asecond-to-last pass to a last pass is 800 to 950° C., a cumulativerolling reduction from the second-to-last pass to the last pass is 10 to40%, and a rolling reduction of the last pass is 8 to 25%; cooling thehot-rolled steel sheet at an average cooling rate of 30° C./s or more ina temperature range of 600 to 720° C., the cooling starting at a time ina range 0.5 to 3.0 sec after an end of the finish rolling; and coilingthe hot-rolled steel sheet at a temperature of 590° C. or lower.
 9. Themethod according to claim 8, further comprising: cold rolling thehot-rolled steel sheet at a cold-rolling ratio of 20% or more to producea cold-rolled steel sheet.
 10. The method according to claim 8, furthercomprising: heating the hot-rolled steel sheet to a temperature of in arange of 730 to 900° C.; and then cooling the hot-rolled steel sheet toa cooling stop temperature of 400 to 590° C. at an average cooling rateof 5 to 50° C./s, wherein the hot-rolled steel sheet is retained for 10to 1,000 sec in a temperature range of 730 to 900° C. for a duration ofthe heating and the cooling, and is retained for 1,000 sec or less in atemperature range of 400 to 590° C.
 11. The method according to claim 9,further comprising: heating the cold-rolled steel sheet to a temperatureof in a range of 730 to 900° C.; and then cooling the cold-rolled steelsheet to a cooling stop temperature of 400 to 590° C. at an averagecooling rate of 5 to 50° C./s, wherein the cold-rolled steel sheet isretained for 10 to 1,000 sec in a temperature range of 730 to 900° C.for a duration of the heating and the cooling, and is retained for 1,000sec or less in a temperature range of 400 to 590° C.
 12. The methodaccording to claim 10, conducted in an atmosphere having a dew point of−40° C. or lower in the temperature range of 730 to 900° C.
 13. Themethod according to claim 11, conducted in an atmosphere having a dewpoint of −40° C. or lower in the temperature range of 730 to 900° C. 14.A method for producing a plated steel sheet, the method comprising:plating the steel sheet obtained by the method of claim
 10. 15. A methodfor producing a plated steel sheet, the method comprising: plating thesteel sheet obtained by the method of claim
 11. 16. A method forproducing a plated steel sheet, the method comprising: plating the steelsheet obtained by the method of claim
 12. 17. A method for producing aplated steel sheet, the method comprising: plating the steel sheetobtained by the method of claim
 13. 18. A method of producing the steelsheet of claim 2, the method comprising: hot rolling a slab having thechemical composition to produce a hot-rolled steel sheet, the hotrolling comprising finish rolling under conditions in which atemperature from a second-to-last pass to a last pass is 800 to 950° C.,a cumulative rolling reduction from the second-to-last pass to the lastpass is 10 to 40%, and a rolling reduction of the last pass is 8 to 25%;cooling the hot-rolled steel sheet at an average cooling rate of 30°C./s or more in a temperature range of 600 to 720° C., the coolingstarting at a time in a range 0.5 to 3.0 sec after an end of the finishrolling; and coiling the hot-rolled steel sheet at a temperature of 590°C. or lower.
 19. The method according to claim 18, further comprising:cold rolling the hot-rolled steel sheet at a cold-rolling ratio of 20%or more to produce a cold-rolled steel sheet.
 20. The method accordingto claim 18, further comprising: heating the hot-rolled steel sheet to atemperature of in a range of 730 to 900° C.; and then cooling thehot-rolled steel sheet to a cooling stop temperature of 400 to 590° C.at an average cooling rate of 5 to 50° C./s, wherein the hot-rolledsteel sheet is retained for 10 to 1,000 sec in a temperature range of730 to 900° C. for a duration of the heating and the cooling, and isretained for 1,000 sec or less in a temperature range of 400 to 590° C.21. The method according to claim 19, further comprising: heating thecold-rolled steel sheet to a temperature of in a range of 730 to 900°C.; and then cooling the cold-rolled steel sheet to a cooling stoptemperature of 400 to 590° C. at an average cooling rate of 5 to 50°C./s, wherein the cold-rolled steel sheet is retained for 10 to 1,000sec in a temperature range of 730 to 900° C. for a duration of theheating and the cooling, and is retained for 1,000 sec or less in atemperature range of 400 to 590° C.
 22. The method according to claim20, conducted in an atmosphere having a dew point of −40° C. or lower inthe temperature range of 730 to 900° C.
 23. The method according toclaim 21, conducted in an atmosphere having a dew point of −40° C. orlower in the temperature range of 730 to 900° C.
 24. A method forproducing a plated steel sheet, the method comprising: plating the steelsheet obtained by the method of claim
 20. 25. A method for producing aplated steel sheet, the method comprising: plating the steel sheetobtained by the method of claim
 21. 26. A method for producing a platedsteel sheet, the method comprising: plating the steel sheet obtained bythe method of claim
 22. 27. A method for producing a plated steel sheet,the method comprising: plating the steel sheet obtained by the method ofclaim 23.